Attaching the gripper without extending qpos?

[benthebear93]

Hi, I am a PhD student trying to use MuJoCo and mink for maniplulation simulation.
Thank you for your work :)

I have been trying to attach the robotiq_2f85 gripper from the mujoco_menagerie to ur5e from your example code.

I was somewhat able to attach the gripper(using weld) but i am getting "Invalid qpos size error" which makes seens as i added the gripper.

ValueError: Error: keyframe 0: invalid qpos size, expected length 21 Element name 'home', id 0
Althought it might not be the issue of mink but is there any clever ways i can use mink only for the manipulator?

[kevinzakka]

Hi @benthebear93, I wouldn't recommend attaching the gripper with a weld constraint. You have a few options:

1. Use the newly introduced MjSpec mechanism to attach the gripper to the arm.
2. Use PyMJCF (dm_control module) to attach it. Read this FAQ for an explanation of how to do it. On top of that, a few other examples in mink leverage this functionality, see for example the iiwa + allegro hand example.

Regardless of the method you choose, dealing with keyframes is a bit finnicky at the moment so I would recommend removing it from the model (here) and just define and use it in python.

[benthebear93]

Thank you for the good advice. I always kind of feel lost when I use MuJoCo because there are so many ways to approach it. I’ll take a look at the links you gave me. Thank you! :) 👍

[benthebear93]

Hi @kevinzakka, I've successfully attached the robotiq_2f85 to the UR5e. I have another question about the VelocityLimit function. Although the robotiq_2f85 has 8 joints, I mostly use a single actuator to control the fingers. However, the VelocityLimit function only checks the joint limits, which results in a warning due to the 8 joints of the 2f85.

While it makes sense for the function to check the gripper's joint limits since it is a joint, it feels somewhat unnecessary. Is there anyway to make it ignore the gripper's joints? solve_ik provides a velocity result with 14 elements, which seems excessive for my case.

[kevinzakka]

Hi @benthebear93, a bit confused about your question. The velocity limit takes in a dictionary, which allows you to pick and choose which joints you want to constrain. You can see an example of this in arm_ur5e_actuators.py:

max_velocities = {
    "shoulder_pan": np.pi,
    "shoulder_lift": np.pi,
    "elbow": np.pi,
    "wrist_1": np.pi,
    "wrist_2": np.pi,
    "wrist_3": np.pi,
}
velocity_limit = mink.VelocityLimit(model, max_velocities)
limits.append(velocity_limit)

[benthebear93]

@kevinzakka Sorry, my mistake. It was actually the check_limits function in the configuration that triggers the WARNING because configuration.qval exceeded the joint limits. Is it normal to see this WARNING when running solve_ik?
WARNING:root:Value 0.00003153 at index 11 is outside of its limits: [-1.57, 0.00]

This doesn’t seem to happen for the UR5e joints, but it frequently occurs with the gripper’s joints. I could adjust the tolerance values to ignore the WARNINGs, but that feels a bit odd.

[benthebear93]

google-deepmind/mujoco#2193

I received an answer to a similar question I asked, so I will close the issue as resolved.